JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container-outer">
  <div id="container-inner">
    <div id="sidebar">
     <p>Боковая панель<br><br><br><br><br><br><br><br><br><br>123</p>
    </div>
    <div id="content">
     <p>Основной контент</p>
    </div>
  </div>
</div>

CSS

#container-outer {
  float:left;
  overflow: hidden;
  background: #eee;
}
#container-inner {
  float:left;
  background: #555;
  position: relative;
  right:75%;
}
#sidebar {
  float: left;
  width: 25%;
  position: relative;
  left: 75%;
}
#content {
  float: left;
  width: 75%;
  position: relative;
  left: 75%;
}